1. Field of the Invention
The present invention relates to a miniature electric motor with reduction worm gear unit and more particularly to a miniature electric motor with reduction worm gear unit used for driving an electric window device of an automotive vehicle.
2. Description of the Related Art
A miniature electric motor with reduction worm gear unit (hereinafter simply referred to as a motor) has been conventionally and extensively used for driving the electric window device, an electric sunroof device or the like. The motor has a motor portion and a reduction worm gear unit for producing an output of the motor portion through the reduction worm gear unit.
Lubricant (mainly, grease) having good wear resistance is used for lubricating worm gears of the reduction worm gear unit.
By the way, the electric window device performs opening/closing operations of a window glass of an automotive vehicle. The motor used in the electric window device requires such reverse rotation proof that the motor is never reversed for burglar proof and security even if an external force is applied in an opening direction to the window glass.
In general, an automotive vehicle is used in a wide range of temperature (for example, -30.degree. C. to +80.degree. C.). Therefore, the motor for the electric window device always requires the reverse rotation proof in this environmental temperature range.
Conventionally, there have been proposed a variety of lubricants having general reverse rotation proof. However, there are almost no lubricants for which the reverse rotation proof of the worm gears is taken into consideration. Namely, in the case where conventional lubricant is used for the worm gears, a transmission efficiency of gears is largely changed when the environmental temperature changes.
For this reason, there is a possibility that the window glass of the automotive vehicle might be opened from the outside by the external force in some environmental temperature range. This is disadvantageous in the aspect of the burglar proof and security. In order to solve this problem, it is necessary to ensure the gear transmission efficiency so that the reverse rotation proof may be always maintained even in the worst environmental temperature range.
Therefore, in the conventional motor, the first countermeasure thereof is that a lead angle of a worm is extremely decreased, or the second countermeasure is that a brake device is installed within an interior of the motor, or the third countermeasure is that mat finishing is effected to rough mesh tooth surfaces of the gears in a mat finish manner to increase a frictional coefficient, thereby maintaining the reverse rotation proof.
However, as in the first countermeasure, if the lead angle of the worm is decreased, an outer diameter of the worm is naturally increased so that it is difficult to miniaturize the motor as a whole. If the brake device is provided as in the second countermeasure, the number of the parts of the motor and the number of the steps for assembly are increased, resulting in increased cost.
The third countermeasure is proposed by the present applicant or assignee (Japanese Patent No. 2636958). The mat finishing for increasing the frictional coefficient of the mesh surfaces of the gears and the maintenance work thereof are required.
Thus, with the first to third countermeasures, since the gear transmission efficiency is decreased so that the reverse rotation proof is always maintained in the environmental temperature range, it is difficult to miniaturize the motor. Also, the conventional methods suffer from the difficulty in temperature characteristics.